1. Field of the Invention
The present invention relates to a quality evaluation method, an optical disc device, recording/reproducing system, and information recording medium suitable for conducting quality evaluation in an optical information reproducing apparatus which reads information by irradiating an optical information recording medium.
2. Background Art
At present, as optical disc which is an optical information recording medium, a CD (compact disc), a DVD (digital versatile disc), a BD (blu-ray disc), and BDXL™ are commercialized, and extensively diffused. Those optical disc medium has various types such as a ROM (read only memory) type which is a read-only type, an R (readable) type which is a write-once type, and an RE (rewritable) type which is a rewritable type.
The recording and read of information on an optical disc is conducted by irradiating an optical disc medium with a laser beam. The information is recorded by forming an area in which a state of a recording film material is changed by a heat of the laser beam. The area in which the state is changed is called “mark”, and an area in which the state is not changed is called “space”. Signals are recorded on a recording layer by the combination of the marks and the spaces. In order to realize such recording, a recording film of the optical disc medium is made of a phase-change material, an alloy of an organic dye and an inorganic material, or an oxide. In production of the information, the recording layer on which the signals have been recorded are irradiated with the laser beam having a power lower than that in recording, and recording signals are read on the basis of a difference in the amount of reflected light between the marks and the spaces.
Because the optical disc is a medium commutative storage, the combination of the optical disc medium and the optical disc device is frequently changed. For that reason, the optical disc device adjusts recording and read according to the present combination. For example, in the recording, a recording power and a light emission waveform used in recording are adjusted according to the medium. Also, in order to put an optical spot at the time of recording and read into an optimal state, an objective lens, a collimator lens, and a lens tilt are also adjusted.
In the optical disc, in order to implement those adjustments, various indexes for evaluating the reading signal quality have been proposed. As general indexes, there are an SER (symbol error rate), a BER (byte error rate), and a bER (bit error rate), which are error probabilities of symbols, bytes, and bits in decode results, which are also used in an optical communication. Also, there is a jitter which is an index for evaluating a temporal fluctuation quantity of a signal to a read clock. Different from the above general index, a large number of evaluation indexes based on the reading signal processing system of the optical disc have been also proposed. For a BD, Japanese Journal of Applied Physics Vol. 39, 2000, pp. 819-823 discloses a limit equalizer jitter, and Japanese Journal of Applied Physics Vol. 45, No. 2B, 2006, pp. 1061-1065 discloses a SAM (sequenced amplitude margin). The limit equalizer jitter is a jitter in results of processing the reading signal by a limit equalizer. The SAM is an index for evaluating an error difficulty of the reading signal waveform on the basis of a target signal waveform and an error signal waveform when the reading signal processing of the PRML (partial response maximum likelihood) is applied to the BD. Also, for BDXL™, US2010/0002556 discloses an i-MLSE (integrated-maximum likelihood sequence error estimation), and US2010/0260025 discloses an L-SEAT (run-length-limited sequence error for adaptive target). The i-MLSE and L-SEAT are indexes in which the distribution of an error quantity of the reading signal waveform to the target signal waveform is quantified by a standard variation. In the optical disc, those indexes are used to evaluate the reading signal quality and implement the adjustment at the time of recording and read.
When data is recorded on the optical disc, and saved for a long period of time, there is a need to periodically evaluate an overall surface of the optical disc as to whether a quality of the recorded data is deteriorated with time, or not. If the signal quality is lower than a given level as an evaluation result, a work for moving the data to a brand-new optical disc is required.